Download Chapter 9 Summary

Chapter 9 Summary
The term energy balance concerns the relationship between energy intake and energy expenditure. When
positive energy balance occurs, energy intake exceeds energy expenditure and body weight generally
increases. When a person is in negative energy balance, energy intake is less than energy expenditure,
and body weight decreases. Adipose tissue serves as the body’s primary energy reserve. The number and
size of adipocytes determine its mass. When body fat increases, adipocytes can increase in size
(hypertrophy) and number (hyperplasia). When body fat decreases, adipocytes decrease in size but not in
number.
Energy intake is influenced by both physiological and psychological factors. Hunger is the basic
physiological need for food, whereas satiety is the physiological response to having eaten enough. Both
are controlled by neuropeptides produced in the brain and by neural and hormonal signals from the
gastrointestinal tract, pancreas, and adipose tissue. Psychological factors such as the appearance, taste,
and aroma of food can also influence the desire to eat, regardless of hunger or satiety. The desire for food
is called appetite, whereas the term food craving refers to the desire for a specific food.
The body expends energy to maintain basal metabolism, for physical activity, and to process food. These
components make up a person’s total energy expenditure (TEE). Basal metabolism accounts for most of
the TEE, and is affected primarily by body size and composition. Energy expended for physical activity is
variable, accounting for 15 to 30% of TEE. Thermic effect of food (TEF) accounts for about 10% of the TEE
and reflects the energy expended to process nutrients after eating. Smaller components of the TEE include
adaptive thermogenesis and nonexercise activity thermogenesis (NEAT). Adaptive thermogenesis is the
energy expended to adapt to temperature changes. NEAT is the energy expended on fidgeting and to
support posture. A variety of methods are used to estimate TEE. Direct calorimetry measures the body’s
heat loss, whereas indirect calorimetry measures the respiratory gases oxygen and carbon dioxide. Other
estimates of TEE are based on the metabolism of stable isotopes of hydrogen and oxygen in the body.
Mathematical formulas called the Estimated Energy Requirements (EERs) can also be used to estimate
TEE.
Overweight is excess weight for a given height, whereas obesity is an abundance of body fat in relation to
lean tissue. Indices such as height–weight tables and body mass index (BMI) are used to assess body
weight. Height–weight tables list “ideal weights” for adults. BMI is based on the ratio of weight to height
squared and is considered a good indicator of body fat. Measures of body composition include
hydrostatic weighing, dual-energy x-ray absorptiometry (DEXA), bioelectrical impedance, near-infrared
interactance, skinfold thickness, computed tomography (CT), magnetic resonance imaging (MRI), and
isotope dilution. Body fat centralized within the abdomen is called android adiposity, whereas body fat
located in the thighs and hips is gynoid adiposity. Body fat distribution is typically assessed by
measuring waist and hip circumference.
Many factors have contributed to the increasing prevalence of obesity in the United States. For example,
average energy intake in adults has increased, and flavorful energy-dense foods are widely available and
affordable. Furthermore, most Americans do not exercise regularly. Thus the combination of increased
energy intake and decreased energy expenditure increases the likelihood of weight gain. Although
lifestyle, socioeconomic, and cultural factors influence body weight, genetics greatly affects our
susceptibility to obesity as well. Scientists have long suspected that internal signals regulate body weight
by adjusting energy intake and expenditure, and developed the “set point” theory to explain this
phenomenon. Research later confirmed that the hormone leptin plays a role in regulating body weight.
However, most obese people produce adequate amounts of leptin. This suggests that leptin may instead
protect against starvation, rather than preventing obesity.
The brain receives information about energy intake and the body’s energy reserve via nerves, circulating
nutrients, and hormones. This complex signaling system strives to maintain energy balance by making
adjustments in energy intake and/or energy expenditure. The brain releases catabolic and anabolic
neuropeptides that promote weight loss and weight gain, respectively. Signals from the GI tract play a
role in regulation of short-term food intake and include gastric stretching and GI hormones. Circulating
concentrations of glucose, fatty acids, and amino acids also influence hunger and satiety. The majority of
GI hormones inhibit food intake with the exception of ghrelin, which stimulates hunger. The hormones
leptin and insulin play a role in long-term regulation of energy balance by suppressing the release of
anabolic neuropeptides in the brain. When leptin and insulin concentrations decrease, anabolic
neuropeptides are released, resulting in overall increased energy intake and decreased energy
expenditure.
Stringent dieting alone rarely results in long-term weight loss. Instead, maintaining weight loss requires
lasting lifestyle changes, which include eating healthy foods and regular exercise. A healthy weight-loss
and weight-maintenance program sets reasonable goals, encourages intake of low–energy-dense,
nutrient-dense foods, and promotes regular physical activity. A realistic weight-loss goal is to decrease
body weight by 5 to 10%, not exceeding 1 to 2 pounds each week. Choosing reasonable serving sizes is
also important for weight loss and weight management. In addition, exercise helps prevent weight gain
and improves overall health.
Some experts recommend low-carbohydrate diets for weight loss, whereas others recommend low-fat
diets. Proponents of low-fat diets believe that less fat in the diet leads to consumption of fewer calories
and to greater weight loss. However, reductions in fat intake have accompanied increased prevalence of
obesity in the United States. In addition, compared to low-fat diets greater short-term weight loss is
achieved on low-carbohydrate diets, but long-term differences have not been found. Advocates of lowcarbohydrate diets believe that high carbohydrate intake can cause insulin levels to rise, leading to
weight gain. In addition, limiting carbohydrates causes the body to go into a ketotic state, which results
in decreased appetite. Low-carbohydrate diets have been criticized for being too high in fat and protein;
however, not all low-carbohydrate diets are high in protein. Concerns regarding the lack of fruits,
vegetables, and whole grains have been also expressed. A positive aspect of weight loss associated with
low-carbohydrate diets is the loss of body fat without substantial loss of lean tissue.